帮我回答一个英语问题

题目

帮我回答一个英语问题
Why should a written contract he signed although the seller and the buyer have reached an agreement through negotiation?
不是，打错了，应该是Why should a written contract be signed although the seller and the buyer have reached an agreement through negotiation?

答案

如果买家和卖家已经通过谈判达成了协议,为什么还需要签合同?